Evolution and Distribution of the Early Aftershocks Following the 2008 Mw 7.9 Wenchuan Earthquake in Sichuan, China

Abstract

Using seismic data recorded by a dense temporary seismic array, we apply a waveform matched-filter technique and obtain an early aftershock catalog around the mainshock rupture zone of the 2008 Wenchuan, Sichuan, China, earthquake. Specifically, we use 1,273 events in a relocated aftershock catalog as templates and scan through the waveforms within 7 days following the mainshock. We obtain up to ∼5.3 and ∼10 times more aftershocks than the standard and relocated catalogs, respectively. We find that early aftershocks mostly occurred below 10 km depth, downdip to large coseismic slip areas. Early aftershocks also show minor along-strike migration with time since the mainshock, indicating triggering of afterslip following the mainshock. From the epicentral region to Beichuan where the mainshock slip was primarily reverse faulting, early aftershocks illustrated the Yingxiu-Beichuan and Guanxian-Jiangyou faults as high-angle listric feature and rooted to a gentle dipping plane at the depth of ∼20 km. The aftershocks in the northeast segment of the Longmen Shan faults mostly occurred along a near-vertical dipping fault, consistent with the primary strike-slip component of mainshock slip in this region. In addition, differences in early and long-term aftershock distribution indicate significant postseismic deformation along the conjugate Xiaoyudong fault. Our results help to illuminate the deep geometry of the Longmen Shan fault zone and the early evolution after the Wenchuan mainshock, which provides additional constraints to the mechanism of plateau uplifting.